Preparation of pure titanium dioxide



Reissued Jan. 14, 1941 UNITED STATES 21,693 PREPARATION OF PURE TITANIUMDIOXIDE Walter W. Plechner, Metuchen, and Arthur W. Hixson, Leonia, N.J., assiglors to National Lead Company, New York, N. Y., a corporationoi. New Jersey No Drawing. Original No. 2,113,946, dated April 12, 1938,Serial No. 109,458, November 6, 1936. Application for-reissue December15, 1939, Se-

rial No. 309,378

5 Claims.

The present invention relates to titanium dioxide pigments. It hasspecial reference to titanium dioxide pigments wherein the titaniumdioxide is of rutile modification.

In our copending application, Serial No. 754,464, filed November 23,1934, we have disclosed a method for preparing titanium dioxide suitablefor use as a pigment and possessing the crystalline structure and formof rutile.

It is believed that the hiding power of a pigment is some function ofthe refractive index among other properties; the greater the differencebetween the refractive index of the pigment and that of the vehicle inwhich it may be suspended the greater will be the hiding power of thepigment. In this way is partially explained the relatively high hidingpower of the titanium pigments. Thus the refractive indices of a few ofthe common white pigments are:

Titanium dioxide which has hitherto been commercially prepared hasalways been precipitated by the hydrolysis of sulphuric acid solutionsof titanium, and, as has been shown by Weiser and Milligan: J. Phys.Chem. 38,513 (1934), such an oxide always exists as the anatasemodification which has the refractive index indicated in the I abovetabulation.

It has been long known that the refractive index of the rutilemodification of titanium dioxide is 2.71, and, therefore, if titaniumdioxide could be so precipitated as to form rutile on calcination, and,furthermore, if the precipitation conditions, or hydrolysis conditionsas it is frequently called in the art, were such as to form particleswhich on calcination were suitable for pigment purposes, a new whitepigment far superior in hiding power to the pigment grade titaniumoxides now commercially available would have been obtained. 7

It, also, was shown by Weiser and Milligan in the publication citedabove thatwhen titanium dioxide is precipitated from a chloride solutionthe rutile modification is always obtained. Titanium dioxide has notbeen prepared under such conditions in the past because until the timeof our discovery it was not known how to precipitate the oxide from achloride solution and obtain the other properties, (purity, whiteness,proper free from impurities contained in the mother liquor. This extremefineness of particle size and difliculty in washing has resulted incalcined products of very poor color and very low hiding power, entirelyunsuitable for use as a pigment.

We have discovered that under suitable conditions, ,to be disclosedbelow, titanium dioxide adapted to pigment uses can be precipitated froma titanium chloride solution. Furthermore, pigmeet so obtained shows aremarkably improved tinting strength and hiding power over titaniumdioxide pigments hitherto offered in the trade. Thus, if tintingstrength is determined by the tentative method of test of the AmericanSociety of Testing Materials, A. S. T. M. Designation: D352-31T, A. S.T. M. Tentative Standards 1933, 525, the highest value shown byoldcommercial products is about 1100-1200, whereas our pigment gives valuesof 1400-1500. The hiding power of our formoi titanium dioxide iscorrespondingly greater. We now ascribe this to the fact that ourpigment is in the rutile modification since it is precipitated from achloride solution, and has, therefore, a higher refractive index thanthe ordinary titanium dioxide pigment.

It may be stated, therefore, that it is one of the primary objects ofthe present invention to provide titanium dioxide which possessestinting strength and hiding power greatly improved over similar types ofpigments hitherto used, and which is readily derived from a titaniumchloride solution.

We have discovered that hydrous titanium oxide may be precipitated froma chloride solution in a form which, upon calcination, yields rutiletitanium dioxide eminently adaptable for use as a pigment if theprecipitation, or hydrolysis, is carried on under such conditions as togive relatively rapid precipitation in the presence of coagulants whichprevent the well-known peptizing action of the chloride ion. Our presentinvention contemplates the use of coagulants which overcome thepeptizing action and permits the hydrous titanium oxide to develop aparticle size requisite for a product designed to be used as a pigment.These coagulants may be broadly defined as polyvalent, negative ionswhich will include diand trivalent negative ions of inorganic acids.aswellasionizableorganic acids.=1"orexample, the citrate, phosphate,oxalate. tartrate. sulfate. arsenate, and when the hydrous titaniumoxide is precipitated in the cold, the carbonate ion, and otherpolyvalent ions behave as coagulants-during the hydrolysis and cause thehydrous titanium oxide to be precipitated in a readily illterable andeasily washed state. The titanium dioxide obtained from the calcinationof hydrous titanium oxide so precipitated has remarkably high hidingpower and tinting strength and is comparable in all other respects tothe titanium pigment of the prior art.

In our aforementioned copending application we have shown a coagulatingeifect of divalent negative ions upon precipitation of hydrous titaniumoxide from chloride solutions. We have .now found that trivalentnegative ions, such as the phosphate and citrate ions are also eifectiveas ooagulents for hydrous titanium oxide precipitated from chloridesolutions.

Hence, one object of the present invention is a method for preparingpigmentary titanium dioxide of the rutile modification. Another objectof our invention is an improved method of utilizing chloride solutionsof titanium in the preparation of titanium pigments. These, and otherobjects of our invention, will be understood from this description.

The coagulating agents of the present invention are effective when usedin small amounts.

Generally, an amount about 1% calculated as weight of the polyvalentnegative radical on the basis of T10: present in the solution, should beemployed. However, amounts as low as 0.05% are effective. It will be.understood by those skilled inthe art that these polyvalent negativeions have a tendency to associate themselves as by adsorption with theparticles of hydrous titanium oxide and are not removed by washing..

' This is particularly true of the sulfate and phosphate ions.Consequently, it will be understood that the amount of coagulating agentto be employed should not be so great as to undesirably contaminate thehydrous titanium onde.

The titanium chloride solution maye be employed as a relativelyconcentrated solution containing about 15%, or thereabouts, of titaniumdioxide and being substantially free from impurities. It will beunderstood that such solutions give optimum results but our invention isnot to be limited to any particular concentration of titanium in thechloride solution nor to any condition purity of the said solution.

The titanium solution may be added to hot water containing the desiredamount of coagulant or the coagulant may be added as an aqueous solutionto the titanium solution. The coagu-'lantmaybeusedintheformoftheacidsofthey respective polyvalent negativeradicals or aqueous sohitionsofsaltsoftheseacidssuchasthealka linesalts, for example, sodium sulfate, ammonium oxalate, lithium carbonate,sodium arsenate, 5

dium citrate. potassium tartrate, etc. A conv ient method of employingthe. coagulant 'to dissolve it in a large volume of water. then to raisethe temperature of the solution and to add thereto the titanium chloridesolution. 1

Thecoagulating agents of the present invention are eifective of how thehydrous titanium oxide is precipitated from the chloride solution. Forexample, "the hydrous titanium oxidemaybeprecipitated-inthewell-hiownmanner'bytherlnalhydroiysisbroughtaboutbyraising the temperature of thesolution to. or approachingtheboiiingpointwhichmayormaynotbeaccompanniedbyadilutionofthetltanium solution. On the other hand,the hydro: titanium oxide maybe precipitated by adding to the titaniinnsolution, at ordinary or even reduced temperature, an alkaline orneutrali xlng agent, such as an alkaline hydroxide or carhmate ofammonia. Strictly speaking, all these methob of precipitating-hydroustitanium oxide maybe regarded as an hydrolysis and accordingly. it willbe understood that when using the term, hydrolytically precipitating inthis description and .in the claims appended hereto, we mean to unbracethese methods. In both cases, the presence of a small amount ofcoagulant permits the precipitation of hydrous titanium oxide in anonpeptized condition and possessing the desirable characteristicsherein set forth.

Having thus described our invention the following examples are given.for illustrative purposes i'rom which, be construed.

Example No. 1.-Titanium tetrachloride is dissolved in two volumes ofcold water yielding a clear aqueous chloride solution of titanium con-.taining about per centtitanium dioxide. All

the iron present is reduced to the ferrous condition in. order toprevent the hydrolysis which ferric salts readily undergo. In orderto-insure the absence of ferric iron throughout the precipitation, weprefer to reduce the solution to a content of two to three grams perliter of titanium dioxide in the tltanous state. Having 1000 pounds ofthis chloride solution containing 15 per cent titanium oxide, this isadded to 7000 however, no limitations are to pounds of boiling watercontaining two pounds ofcitric acid, during about one hour. When theaddition has been completedv about 95 per cent of taining 10 pounds ofoxalic acid during about one,-

hour. When the addition has been completed about 95 per cent of thetitanium will have been precipitated as hydrous titanium dioxide: the

latter is in a coagulated form such that it settles well andmay bereadily filtered and washed. After nitration and washing the precipitateis converted to the anhydrous rutile form by calcining at a temperatureof from 700' to 1000 C.

The foregoing description of our invention has been given for clearnessof understanding and no undue limitations should be deduced them frombut the appended claims should be interpreted as broadly as possible inview o! the prior We claim:

1. Process for the preparation of titanium dioxide having rutilecrystalline 'structurewhich comprises hydrolytically precipitating non-DP- tiaed. easily fllterable, hydrous titanimn oxidetainingasmallamountofanegativml flllmt coagulant ion, separating thesaid-hydrous titanium oxide and calcining it to rutile crystal line froman aqueous titanium chloride solution contanium oxide and calcining itto rutile crystalline structure.

3. Process for the preparation of titanium dioxide having rutilecrystalline structure which comprises hydrolytically precipitatingnon-peptized, easily fiiterable hydrous titanium oxide from an aqueoustitanium chloride solution con.-'

taining a small amount of. a co llflant selected from the groupconsisting of the acids and alkali metalsalts or the phosphate andcitrate radicals,

separating the said hydrous titanium oxide and calcining it to rutilecrystalline structure.

4. Process for the preparation oi titaniumdioxide having rutilecrystalline structure which comprises hydrolytically precipitatingnon-peptized, easily fllterable hydrous titanium oxide from an aqueoustitanium chloride solution containing a small amount. or citric acid,separating the said hydrous titanium oxide and calcining it to rutilecrystalline structure. 5. Process for the preparation of titaniumdioxide having rutile crystalline structure which compriseshydrolytically precipitating non-peptized, easily fllterable hydroustitanium oxide from an aqueous titanium chloride solution containinga'small amount of phosphoric acid. separating the said hydrous titaniumoxide and calcining it to rutile crystalline structure.

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